The HIV envelope gp41 is a heavily glycosylated transmembrane protein. The difficulty in producing full length recombinant gp41 necessitates an incremental approach for structural determination studies. The ectodomain region located on the outer surface of the viral membrane directly mediates membrane fusion events via an N-terminal fusion domain or peptide (FP). The structure of a truncated gp41 ectodomain (residues 27-154) lacking FP and the membrane anchor is a rod-like trimer comprising three parallel N-terminal alpha-helices assembled as a coiled-coil in the center with three antiparallel C-terminal alpha-helices packed on the outside with highly flexible loops connecting the inner and outer helices. This so called 6-helical bundle (6HB) is the fusion active conformation. There have been no direct structural determinations on the prefusion conformation where the N- and C-terminal helical regions are extended (pre-hairpin intermediate). The blocking or inhibition of the transition from the pre-hairpin to 6H conformation is a major target for anti-HIV drug targeting. To understand in more detail the interactions between all the gp41 domains, constructs were made which included the N-terminal FP and the membrane anchor. This complex membrane protein (gp41 residues 1-194) was expressed in bacteria and purified as a protein-detergent complex. High resolution NMR techniques are being used to determine the structure and conformational flexibility of gp41. This is a major undertaking given the high molecular mass of the protein - detergent complex (over 100,000). Detailed NMR measurements reveal a high degree of intrinsic mobility for the homotrimeric HIV-1 gp41. The fusion peptide (FP) exhibits high amplitude motion on the fast nanosecond time scale. The linker between the N and C-terminal helical regions shows both fast and slow (microsecond) motions. The findings are compatible with the protein switching rapidly between the pre-hairpin intermediate, three-helical bundle state, and the late-fusion, anti-parallel 6HB. Structural studies by NMR are also being carried out to map interaction sites of various drug and antibodies with potential therapeutic value. With the current improvements made in the protein preparations, we are also reexamining condition for protein crystallization. These structural studies will provide insight into the fusion mechanism and so provide direction for targeted anti-HIV intervention.